Add strcpy function

README.md

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+# LibAsm
+This not-so-awfully-creatively-named repository is a collection of various common routines and data abstractions,
+the kind you might find in a standard C library, however written in assembly.
+
+It is *not* an implementation of libc, or any other standard C library, because I want to have the freedom to implement everything in my own way, face the challenges of creating my own abstractions, and perhaps have the ability to cut my own corners as well :)
+
+Each function will be available as its own file, intended to make it easy to import only the ones you need within a project.
+The overall ambition is to be able to use this library to losslessly abstract the most-optimal instructions one would normally write by hand - but with an easier manner of use, e.g. providing macros and functions to do everything that's required.
+
+Ultimately, it may be impossible to fully equal "the most optimal code" when using functional calls and generic abstractions, since each problem may have more optimal corner-cuttings which result from eliminating the parts that aren't necessary from the generic implementation, but without having to worry about any usual C-related standards, we can still perhaps make a good crack at the problem nevertheless!
+
+## Usage
+To compile a particular file to an object:
+```bash
+gcc -c -g strcpy.S
+```
+To then link to an executable (only if there is a \_start symbol defined somewhere):
+```bash
+ld strcpy.o
+```
+WIP: I am still working on specifying a custom entry point so we don't have to play along with GNU Assembler's desires.
+

alloca.S

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+.intel_syntax noprefix
+
+.macro  alloca size
+sub rsp, \size
+.endm
+
+.text
+
+.globl _start
+
+_start:
+    # Could just use push, but this also works.
+    alloca 16
+    mov byte ptr [rsp+15], 10
+    mov byte ptr [rsp+14], 33
+    mov byte ptr [rsp+13], 100
+    mov byte ptr [rsp+12], 108
+    mov byte ptr [rsp+11], 114
+    mov byte ptr [rsp+10], 111
+    mov byte ptr [rsp+9], 119
+    mov byte ptr [rsp+8], 32
+    mov byte ptr [rsp+7], 44
+    mov byte ptr [rsp+6], 111
+    mov byte ptr [rsp+5], 108
+    mov byte ptr [rsp+4], 108
+    mov byte ptr [rsp+3], 101
+    mov byte ptr [rsp+2], 72
+
+# Can't move imm64 to memory directly, so have to copy to register first!
+#    mov rax, 729975031549884192
+#    mov qword ptr [rsp+8], rax
+#    mov rax, 48857072035144
+#    mov qword ptr [rsp], rax
+
+    mov rax, 1
+    mov rdi, 1
+    lea rsi, [rsp]
+    mov rdx, 15
+    syscall
+
+    mov rax, 60
+    mov rdi, 0
+    syscall

strcpy.S

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+    .intel_syntax noprefix
+
+    .globl _start
+
+    .text
+
+
+# String-copying routine.
+# Commented-out lines would also return the starting address of the destination address.
+strcpy:
+#    push rsi
+.loop:
+    movb r10b, [rdi]
+    cmpb r10b, 0
+    je .end
+    movb [rsi], r10b
+    inc rdi
+    inc rsi
+    jmp .loop
+.end:
+#    pop rax
+    ret
+
+_start:
+
+    mov rdi, offset str2
+    mov rsi, offset str1
+    call strcpy
+    # mov rsi, rax  -- would copy the string start address to rsi
+
+    mov rax, 1
+    mov rdi, 1
+    mov rsi, offset str1
+    mov rdx, 14
+    syscall
+
+    # exit
+    mov rax, 60
+    mov rdi, 0
+    syscall
+
+    .bss
+str1:
+    .skip 30
+
+    .data
+str2:
+    .string "Hello, world!\n"